How to ADD COLUMN in SQL Server

Galaxy Glossary

How do I add a column to an existing SQL Server table?

ADD COLUMN adds one or more new columns to an existing SQL Server table without affecting existing data.

Sign up for the latest in SQL knowledge from the Galaxy Team!

Description

What does ADD COLUMN do in SQL Server?

ADD COLUMN, used inside an ALTER TABLE statement, appends a new field definition to an existing table. The command leaves current rows intact and sets the new column to NULL or the supplied DEFAULT value.

How do I write the basic ADD COLUMN statement?

Write ALTER TABLE table_name ADD column_name data_type [NULL | NOT NULL] [DEFAULT value]. Finish the statement with a semicolon.

Can I add multiple columns at once?

Yes. List each definition after a single ADD keyword, separated by commas. This applies the change in one transaction and avoids multiple table locks.

Which options are most useful?

Common options include NOT NULL, DEFAULT, IDENTITY, COLLATE, and inline CHECK or FOREIGN KEY constraints. Enterprise Edition supports ONLINE = ON for reduced locking.

How can I avoid blocking in production?

Add the column as NULLable first, backfill values in batches, then ALTER the column to NOT NULL. Use shorter transactions and off-hours windows when possible.

Example: Add delivery_status to Orders

The sample query below shows how to add a VARCHAR(20) column with a DEFAULT and update existing orders in one step.

What are the best practices?

Always test on staging, script a rollback (DROP COLUMN), and monitor execution plans. Include IF NOT EXISTS to make deployments idempotent in SQL Server 2022+

Why How to ADD COLUMN in SQL Server is important

How to ADD COLUMN in SQL Server Example Usage


-- Add delivery_status to Orders and set a default value
ALTER TABLE Orders
ADD delivery_status VARCHAR(20) NOT NULL DEFAULT ('pending');

How to ADD COLUMN in SQL Server Syntax


-- Add one column
ALTER TABLE [schema_name.]table_name
ADD [COLUMN] column_name data_type
    [NULL | NOT NULL]
    [DEFAULT default_value]
    [IDENTITY [(seed,increment)]]
    [COLLATE collation_name]
    [CONSTRAINT constraint_name { PRIMARY KEY | UNIQUE | CHECK (expression) | REFERENCES ref_table(column) }];

-- Add multiple columns at once
ALTER TABLE table_name
ADD column_a INT NULL,
    column_b VARCHAR(50) NOT NULL DEFAULT ('N/A');

Common Mistakes

Adding a NOT NULL column without a DEFAULT on a table that already has rows. SQL Server throws an error because existing rows would violate NOT NULL. Fix: add the column as NULL, backfill, then ALTER to NOT NULL or include a DEFAULT.
Running separate ALTER TABLE statements for each new column, which locks the table multiple times. Fix: add all new columns in one ALTER TABLE … ADD block, separated by commas.

Frequently Asked Questions (FAQs)

Is ADD and ADD COLUMN the same in SQL Server?

Yes. SQL Server treats ADD and ADD COLUMN identically. COLUMN is optional and often omitted in day-to-day scripts.

Can I add a column with a foreign key in one step?

Yes. Provide CONSTRAINT fk_name REFERENCES parent_table(parent_id) directly after the column definition. SQL Server builds the key during the ALTER TABLE.

Does ADD COLUMN lock the entire table?

Depending on edition and options, SQL Server may hold schema modification (SCH-M) locks. Enterprise Edition supports ONLINE = ON to make the change with shorter blocking periods.

Want to learn about other SQL terms?

dbt tests

dbt tests are declarative assertions in dbt that automatically validate data quality during model builds.

dbt utils

dbt utils is a community-maintained dbt package that adds time-saving utility macros for analytics engineers, such as generating surrogate keys, safely unioning models, and automating date spines.

dbt Tutorial Glossary: Build, Test & Deploy SQL Models

dbt is an open-source framework that lets data teams transform raw warehouse tables into tested, documented, production-ready datasets using version-controlled SQL.

Trusted by top engineers on high-velocity teams

Assort Health

Curri

Rubie